230 Mr Chree, On some experiments on [June 1, 



vapour to possess the rare, if not unique, property of showing 

 no electrification. This of course fits in extremely well with 

 the electrolytic theory, which these writers seem to favour. 



Gassiot 1 more than thirty years ago made some interesting 

 experiments on mercury. His apparatus was so constructed that 

 he could have for his electrodes either metal wires or the Hg. 

 surface itself. Describing the appearance of the discharge as 

 the mercury rose in the tube, he says, " as soon as the mercury 

 ascends above the negative wire, a beautiful lambent bluish 

 white vapour appears to arise, while a deep red stratum becomes 

 visible on the surface of the mercury," p. 4. This red glow 

 was only sometimes apparent. In one place Gassiot suggests it 

 may be due to impurities in the mercury, in another he considers 

 it analogous to the glow on a platinum wire electrode, but he 

 seems to have arrived at no final conclusion. A mercury surface, 

 he says, when cathode is all covered with a luminous white 

 glow, but when anode only the extreme point is luminous. He 

 records numerous observations on striae in tubes in which 

 mercury was present, but it is not always easy to follow the 

 exact conditions of the experiment. He mentions that by re- 

 ducing the temperature of a vacuum tube containing mercury to 

 — 102° F. or raising it to 600° F. he caused the striae to disappear. 

 He also on several occasions got rid of striae at ordinary tem- 

 peratures by using for cathode the surface of the mercury itself. 

 Thus on p. 7, Phil. Trans. 1858, he says, "...immediately it 

 (the mercury) covers the negative wire the stratifications dis- 

 appear, and the interior of the globe is filled with bluish light." 

 I think one may fairly conclude from his experiments that the 

 nature of the electrodes and especially of the cathode exercises 

 an important influence on the phenomena observed in the dis- 

 charge through mercury vapour. 



In the experiments now to be described I used the secondary 

 current from an induction coil, varying the primary current ac- 

 cording to circumstances. When the resistance in the vacuum 

 tube circuit is sufficiently large only the direct extra current 

 passes. The appearance of the discharge shows at once whether 

 this is the case. 



The tubes which I employed were constructed by Professor 

 Thomson's assistant, Mr Everett, who rendered me valuable assist- 

 ance in the course of the experiments. As the first tube had 

 only a brief existence it will suffice to describe the second which 

 resembled it in all essential points. The diagrammatic sketch, 

 fig. 1, p. 231, will explain the general character. The nature 

 of the electrodes varied but their position in the tube was fairly 



1 Phil. Trans. 1858, pp. 1—16; and 1859, pp. 137—160, 



